A method for assay of the activity of mevalonate pyrophosphate decarboxylase in fibroblasts was developed. In similarity to earlier findings with mevalonate kinase, the activity of mevalonate pyrophosphate decarboxylase was increased when whole serum was replaced with lipid-deficient serum in the incubation medium. Unlike mevalonate kinase, mevalonate pyrophosphate decarboxylase activity was not increased by insulin.